Collagen: A fibrous protein that is located in the extracellular matrix of connective tissues. It helps provide a structural framework for both soft and calcified connective tissues. Collagen occurs in the dermis (skin) and is the principal protein in the dental structures, including the teeth, cementum, and alveolar. It occurs in the white fibers of connective tissues like the deep fascia of muscle and in the matrix of bones, cartilage, and tendons. The most abundant animal protein.